1. Field of the Invention
The present invention relates to wind turbines, and particularly to a drive system for a wind turbine with a contra-rotating generator, i.e., a generator having a magnetic rotor and an armature rotating in opposite directions.
2. Description of the Related Art
The use of wind turbines as a source of electricity has been gaining in popularity over the years, since wind turbines utilize wind to produce electricity instead of burning fossil fuels that produce large amounts of pollution that can adversely affect our health and the environment. One type of widely used turbine is the single rotor horizontal axis wind turbine that typically includes a single rotor having a plurality of blades and a generator having a magnetic rotor member that rotates in the field of a stator fixed to the generator housing. Since the single rotor horizontal axis wind turbine only has a single rotor, it can only utilize wind energy coming from a single direction. A significant portion of wind energy, including the wind flowing downstream of the rotor, therefore remains unused. Also, having a single rotor can typically only produce a relatively small rotational speed, which results in a minimal amount of electricity. To compensate for the relatively small rotational speed generated by the single rotor wind turbines, manufacturers typically increase the span of the rotor blades to provide more torque and power capture.
This, however, can require a larger generator design, which, in turn, can result in an excessively heavy wind turbine, costly construction, as well as increased bearing friction. Further, during periods of gusty winds, large torque can be transmitted to the bearing and support structure, thereby shortening the life span of the wind turbine.
The issues resulting from the single rotor wind turbine have sparked renewed interest in the dual counter-rotating wind turbine, in particular, the counter-rotating horizontal axis wind turbine (CR-HAWT). In this type of wind turbine, a contra-rotating generator replaces the unidirectional rotor of the single rotor wind turbines. An upwind rotor is directly fixed to the magnetic rotor member, and a downwind rotor is directly fixed to the armature rotor member, which are coaxial. The contra-rotating wind generator captures an additional portion of the otherwise wasted wind energy from the tangential air downstream from the upwind rotor and also increases the amount of mechanical energy captured due to the increase in the relative rotational speed within the gap between the magnetic rotor and the armature rotor. However, the contra-rotating generator can produce a significant amount of unnecessary vibration. Further, a sudden increase in torque resulting from gusty wind conditions subjecting the upwind rotor to a large amount of torque while the downwind rotor lags behind, which can also result in catastrophic failure.
Thus, a drive system for wind turbine with contra-rotating generator solving the aforementioned problems is desired.